1. Field of the Invention
The present invention relates to an identification tag, and more particularly to a radio frequency identification (RFID) tag using a monopole antenna.
2. Description of the Related Art
FIG. 1 is a schematic diagram of an RFID tag disclosed in U.S. Pat. No. 6,914,562. The conventional RFID tag 1 includes a packaging wall 11, two conductive tabs 12, an RFID device 13, and a radio frequency reflecting structure 14. The conductive tabs 12 and the RFID device 13 are disposed on a surface of the packaging wall 11, and the RFID device 13 is electrically connected to the conductive tabs 12. There are two feed points 15 and 16 between the RFID device 13 and the conductive tabs 12. The radio frequency reflecting structure 14 is disposed on the other surface of the packaging wall 11. In the conventional RFID tag 1, the radio frequency reflecting structure 14 is a metal plate.
The impedance of the conventional RFID tag 1 varies by means of adjustment of the positions of the conductive tabs 12 and the feed points 15 and 16. The conventional RFID tag 1 is disposed on different package structures or containers, but a readable distance of the conventional RFID tag 1 is not improved.
The conventional RFID tag 1 is applicable to a metal object to be measured and is flatly disposed on a surface of the object to be measured. As known in common practice, in order to prevent the conventional RFID tag 1 from approaching the metal too closely and reducing the radiation efficiency, the packaging wall 11 (a dielectric material) may have a certain thickness, so as to reduce the interference with the metal object to be measured; otherwise, the real part impedance in the Smith Chart (an impedance chart in a complex coordinate system) will be too small to emit an electromagnetic wave efficiently. Furthermore, the specification of U.S. Pat. No. 6,914,562 explicitly discloses that, at UHF frequencies, defined as a band in the range of 860 MHz to 950 MHz, a dielectric thickness of about 3 millimeters to 6 millimeters of the packaging wall 11 is required, so the entire thickness is quite substantial and the production cost is rather high (i.e., requiring more materials), and meanwhile, the readable distance is not improved.
Therefore, there is a need for an RFID tag using a monopole antenna that meets the novelty and inventiveness requirements and solves the above problems.